[sankalpmittal]

See: http://postimg.org/image/gtlq9urwb/

See question 62..

I think the order is,

IV>III>II>I, due to decreasing resonance, but its wrong.

How ?

Please help !!

Thanks...

[Babcock_Hall]

I would think about aromaticity.

[camptzak]

 

[sankalpmittal]

I would think about aromaticity.

Ok so IV and II are aromatic but I and III are not.

So IV and II are more stable than I and III.

IV>II>...

Out of I and III which is more stable ? Both are not aromatic.

[kriggy]

I would say its 4>2>3>1. 3 has resonance, 1 has not

[sankalpmittal]

I would say its 4>2>3>1. 3 has resonance, 1 has not

But the textbook say that

IV>II>I>III..

Is the reason, anti-aromaticity ? I mean III is "anti-aromatic" ?

[TwistedConf]

Is the reason, anti-aromaticity ? I mean III is "anti-aromatic" ?

Yes.

[sankalpmittal]

Is the reason, anti-aromaticity ? I mean III is "anti-aromatic" ?

Yes.

But some say that order is IV>II>III>I... Which is correct ?

[camptzak]

Is the reason, anti-aromaticity ? I mean III is "anti-aromatic" ?

Yes.

But some say that order is IV>II>III>I... Which is correct ?

the textbook, anti aromatic compounds are very unstable

[sankalpmittal]

Is the reason, anti-aromaticity ? I mean III is "anti-aromatic" ?

Yes.

But some say that order is IV>II>III>I... Which is correct ?

the textbook, anti aromatic compounds are very unstable

As per my another professor and here : http://www.sciencemadness.org/talk/viewthread.php?tid=24831:

According to JACS :

Antiaromaticity only means the planar cyclic structure is less stable than the nearest open ring structure (e.g., cyclopentadienyl carbocation vs. divinylmethyl carbocation). Nothing more than that.

According to JACS, 89, 1112–1119 (DOI: 10.1021/ja00981a015), the cyclopentadienyl carbocation forms easily from cyclopentadienyl alcohol with strong acids even at cryogenic temperatures. The same cannot be said for cyclohexanol. This is good enough indication that the cyclopentadienyl carbocation might be more stable than the cyclohexyl carbocation.

[camptzak]

I would still say

4>2>1>3

but I think maybe you should do an experiment, it would be fun.

[orgopete]

As per my another professor and here : http://www.sciencemadness.org/talk/viewthread.php?tid=24831:

According to JACS :

Antiaromaticity only means the planar cyclic structure is less stable than the nearest open ring structure (e.g., cyclopentadienyl carbocation vs. divinylmethyl carbocation). Nothing more than that.

According to JACS, 89, 1112–1119 (DOI: 10.1021/ja00981a015), the cyclopentadienyl carbocation forms easily from cyclopentadienyl alcohol with strong acids even at cryogenic temperatures. The same cannot be said for cyclohexanol. This is good enough indication that the cyclopentadienyl carbocation might be more stable than the cyclohexyl carbocation.

This is a very good and instructive problem. Certainly, I don't know the answer. If for a test, I'd go with your professor. However, the link suggests there may be more to this. I had been skeptical of anti-aromaticity, as it seemed more empirical than theoretical. (I am judging by the use of Frost circles.) I had sought verification of the instability of cyclopentadienyl cation, which as I recall is quite unstable. Anti-aromaticity seemed a good predictor of stability, though I had occasionally found others that were not so sure of its rigor.

It would be my opinion that the rules of aromaticity may be good predictors of compound stability, but as the referred JACS paper indicates, we should remain skeptical of whether a truly repulsive (i.e., high energy) interaction actually occurs. If electron donation to an incipient carbocation were a high energy process as predicted by the rules of aromaticity, then electron donation should be disfavored. The result should be a very slow or no reaction. Assuming the citation is correct, the data indicates a quite different result.

Kudos for the enlightment.

[sankalpmittal]

As per my another professor and here : http://www.sciencemadness.org/talk/viewthread.php?tid=24831:

According to JACS :

Antiaromaticity only means the planar cyclic structure is less stable than the nearest open ring structure (e.g., cyclopentadienyl carbocation vs. divinylmethyl carbocation). Nothing more than that.

According to JACS, 89, 1112–1119 (DOI: 10.1021/ja00981a015), the cyclopentadienyl carbocation forms easily from cyclopentadienyl alcohol with strong acids even at cryogenic temperatures. The same cannot be said for cyclohexanol. This is good enough indication that the cyclopentadienyl carbocation might be more stable than the cyclohexyl carbocation.

This is a very good and instructive problem. Certainly, I don't know the answer. If for a test, I'd go with your professor. However, the link suggests there may be more to this. I had been skeptical of anti-aromaticity, as it seemed more empirical than theoretical. (I am judging by the use of Frost circles.) I had sought verification of the instability of cyclopentadienyl cation, which as I recall is quite unstable. Anti-aromaticity seemed a good predictor of stability, though I had occasionally found others that were not so sure of its rigor.

It would be my opinion that the rules of aromaticity may be good predictors of compound stability, but as the referred JACS paper indicates, we should remain skeptical of whether a truly repulsive (i.e., high energy) interaction actually occurs. If electron donation to an incipient carbocation were a high energy process as predicted by the rules of aromaticity, then electron donation should be disfavored. The result should be a very slow or no reaction. Assuming the citation is correct, the data indicates a quite different result.

Kudos for the enlightment.

Thanks a lot !!

More comments are welcomed..

[Dan]

This paper will also be of interest:

Lossing & Traeger JACS 1975, 97, 1579

[Corribus]

Don't have much to add to this discussion except a brief comment on this:

I had been skeptical of anti-aromaticity, as it seemed more empirical than theoretical.

Considering it came out of a theoretical framework for pi-stabilization energy, I'm not sure how it qualifies as empirical rather than theoretical.  A simple Huckel treatment of cyclobutadiene yields the prediction that it is significantly less stable (~30 or so kcal/mol) than linear butadiene (ignoring bond strain), and that it is a diradical.  Conjugated ring systems (one ring, mostly) which are not closed shell and stable are generally predicted to be the case with the number of conjugated electrons not equal to 4n + 2.  Hence Huckel's rule.  Hence the idea of "anti-aromaticity"*.  You can show a similar effect with other simple theoretical treatments of conjugated pi electrons.  True, these models are rather simplistic and there are certainly exceptions, but it's still a concept arising from a theoretical model of pi electron stabilization.

* I do think the terminology is kind of strange (what does it mean to be "against" or "opposite" of aromaticity?) and have never particularly liked it.

(Full disclosure - haven't read the articles linked to above.  Going only from my research experience with the physics of cyclic aromatic systems.)